Waveguide arrays are used at radio and optical wavelengths. In particular they are used at optical wavelengths to realize imaging lenses and gratings in wavelength routers. One such wavelength grating router is described in U.S. Pat. No. 5,136,671, issued on Aug. 4, 1992. As shown in FIG. 1, this router is an imaging arrangement formed by a waveguide grating and it is generally composed of two dielectric slabs 101 and 102, two periodic arrays 103 and 104, and a set of waveguides (grating arms) 105, of different lengths between the two arrays. Typically, in commercial routers the loss exceeds 3 db for the central ports of the router and, for the marginal ports it often exceeds 6 db. Such losses seriously limit the usefulness of the router for passive networks and applications such as channel dropping filters with stringent requirements on loss uniformity. It is generally important to improve uniformity, by reducing the difference between maximum and minimum loss. The above loss variation is primarily caused by the two periodic arrays connected to the waveguides forming the grating. It is caused by scattering at the two junctions where the radial waveguides of the two periodic arrays connect to the input and output slabs. Efficient router arrays are currently realized by means of transitions that are difficult to realize with low loss because this would require very small gaps 110, between adjacent waveguides. Thus, there is continuing need to reduce this loss and its uniformity. An important result, which is obtained by reducing the loss, is that the array element pattern then approaches a rectangular function. Because of this, the array can then be used to realize rectangular transfer functions in a wavelength slicer.